(1) Field of the Invention
The present invention relates to a planar antenna. The invention relates particularly to an art suitable for use as an antenna which is formed on a dielectric substrate to generate circularly polarized waves.
(2) Description of the Related Art
Recently, vehicles (movable objects) such as automobiles are often equipped with antennas for high-frequency band GPS (Global Positioning System) and antennas for receiving satellite radio waves of satellite digital broadcasting. In addition, there is a need for antennas for transceiving radio waves in ETC (Electronic Toll Collection) system, which automatically collects tolls for express ways and toll roads, and radio beacons in VICS (Vehicle Information Communications System), which provides traffic information.
Of such radio waves to be transceived by movable objects, circularly polarized waves are used in GPS radio waves, satellite radio waves for satellite broadcasting, and ETC radio waves. Most of the previous antennas for circularly polarized waves are patch antennas (planar antenna).
FIG. 11 is a schematic plan view showing a construction of an example of a previous planar antenna, and it is disclosed in the following patent document 1. The planar antenna of FIG. 11, which is for receiving right-hand circularly polarized waves, includes a square-like loop antenna (power-fed element) 120 and a linear electric conductor [parasitic (non-power-supplied) element] 140 mounted on a dielectric (transparent film). The linear electric conductor 140, which is an independent conductor not coupled to the loop antenna 120, is bent to be divided into two parts, a first part 140A and a second part 140B. Reference characters 160 and 170 designate power-feeding terminals for supplying the loop antenna 120 with electric power; reference character 270 designates connecting conductors which connect power-feeding terminals 160 and 170 to the loop antenna 120; reference character CP designates the center point of the loop antenna 120.
As shown in FIG. 11, the parasitic element 140 is placed outside the loop antenna 120 and is arranged close to the loop antenna 120. In more detail, the first part 140A is placed in parallel with one side of the loop antenna 120; the second part 140B is placed in parallel with a straight line which connects an intermediate point between the power-feeding terminals 160 and 170 and an apex of the loop antenna 120 which is opposite the intermediate point.
Referring to paragraph [0069] of the following patent document 1, a description will be made hereinbelow of the parasitic element 140. A loop antenna 120 without a parasitic element 140, in particular, a loop antenna 120 whose circumference (the total length of the antenna conductor) is equal to one wavelength, can receive only an electric field component (lateral component) in the horizontal direction (that is, it is impossible to completely receive circularly polarized waves in which the direction of the electric field changes over time). The parasitic element 140 arranged close to the loop antenna 120 makes it possible for the loop antenna 120 to receive a vertical component of the circularly polarized waves.
That is, the second part 140B of the parasitic element 140 takes in the vertical component of the circularly polarized waves, and this received vertical component is coupled to the antenna conductor of the loop antenna 120 by the first part 140A which is close to the antenna conductor of the loop antenna 120. As a result, the vertical and lateral components of the circularly polarized waves are received by the loop antenna 120 in phase. In other words, with only the second part 140B, it is difficult to transfer the received circularly polarized waves to the loop antenna 120. Thus, in order to efficiently transfer the received circularly polarized waves to the loop antenna 120, the parasitic element 140 is provided with the first part 140A.
Further, other previous antenna construction are disclosed in the following patent documents 2 and 3.
Patent document 2 relates to a thin and flat antenna construction including more than one stacked loop antenna element. The antenna of patent document 2 is capable of generating left-hand circularly polarized waves and right-hand circularly polarized waves at the same time from two directions.
Patent document 3 relates to an antenna construction in which a large square row antenna is provided in the plane of an antenna. Inside the large antenna, a small dipole antenna, a loop antenna, and a planar antenna are arranged so that the directivities of the antennas formed by interference of the antennas are optimum.
[Patent document 1] Japanese Patent Application Laid-open No. 2005-102183
[Patent document 2] Japanese Patent Application Laid-open No. 2005-72716
[Patent document 3] Japanese Patent Application Laid-open No. HEI 9-260925
However, the art disclosed in patent document 1 is disadvantageous in that electric field distribution to the parasitic element 140 is weak due to the antenna construction, so that it is difficult to obtain a sufficiently good circular polarization characteristic. This is probably because a linear antenna (e.g., a dipole antenna) simply mounted on a dielectric substrate generates a beam in the direction along the surface of the dielectric substrate, so that the intensity of radiation in the direction (that is, the direction along the thickness) crossing the surface of the dielectric substrate is weak.
Here, the purpose of the art of patent document 2 is generating left-hand and right-hand circularly polarized waves at the same time. In patent document 3, it is possible to place multiple antennas closely or concentratedly in a narrow area, and thus down-sizing is available, and the purpose of the invention is to prevent noise from inside automobiles. Therefore, neither of the applications aims at obtaining a good circular polarization characteristic.